This application is based on an application No. 11-339549 filed in Japan, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a signal processing system and a signal processing method for determining expiration of the life of a cutting tool to be used for cutting. Particularly, the invention relates to a system and a method for processing a signal obtained from an abrasion sensor provided on a cutting edge of a cutting tool.
2. Description of Related Art
Conventionally, an in-process tool abrasion diagnosis for detecting the abrasion and breakage of a cutting tool is based on indirect monitoring methods as described below:
(1) The abrasion is estimated on the basis of a change in a power or a cutting sound observed during a cutting process performed by a machine tool fitted with the cutting tool.
(2) A sensor for sensing an acoustic emission (AE) is provided adjacent a cutting edge of the cutting tool or on a tool holder, and occurrence of breakage and chipping is detected on the basis of an abnormal signal detected by the AE sensor.
In these indirect monitoring methods, the estimation of the abrasion is based on incidental physical phenomena such as the changes in the cutting force and the cutting sound and vibrations which occur incidentally to the abrasion and breakage of the cutting tool. Therefore, the indirect monitoring methods are less reliable with a poor measuring sensitivity. Further, the expiration of the life of the cutting tool is estimated at a lower level of accuracy, because the measurement results tend to include noises due to interference or the like.
One approach to these problems is described in Japanese Unexamined Utility Model Publication No. 3-120323 (1991). This publication discloses a throw-away tip having a sensor line of a conductive film provided along a cutting ridge on a flank thereof. It is also disclosed that the sensor line has a width conforming to an allowable abrasion width. In accordance with the throw-away tip disclosed in the publication, the sensor line is worn as the cutting ridge is worn, so that the expiration of the life of the cutting ridge can be detected when the sensor line is cut off.
Further, Japanese Unexamined Patent Publication No. 9-38846 (1997) proposes an ordinary cutting tool (not a throw-away tip) which has a thin film circuit on a flank thereof, wherein the expiration of the life of the cutting tool is automatically detected by sensing a change in electrical resistance which occurs due to abrasion of the thin film circuit as the flank is worn.
For the in-process tool diagnosis, the aforesaid method is preferred in which the sensor line is provided along the cutting ridge of the cutting tool to sense the change in the electrical resistance of the sensor line for the detection of the abrasion of the cutting ridge.
However, a resistance signal obtained from the sensor line during the cutting process is a fairly unstable signal which contains a noise and erroneous data. Therefore, it is important to extract a true signal component indicative of the abrasion of the sensor line from the resistance signal for determining the abrasion degree and breakage such as chipping of the cutting tool on the basis of the resistance signal from the sensor line.
To solve the aforesaid problems, it is an object of the present invention to provide a signal processing system and a signal processing method which are adapted to perform a signal processing operation on measurement data (sensor data) obtained from a cutting tool with an abrasion sensor to provide highly reliable process data.
In accordance with the present invention, a cutting tool has an abrasion sensor provided on a cutting edge thereof for detecting abrasion and breakage thereof occurring due to a cutting operation, and a resistance signal from the sensor which varies depending on the abrasion degree of the cutting tool is captured as a relatively roughly and discontinuously changing signal for determining the abrasion degree of the cutting tool at a discontinuous change point.
More specifically, the present invention has the following first to seventh inventive aspects.
In accordance with a first inventive aspect, there is provided a signal processing system for a cutting tool having an abrasion sensor provided on a cutting edge thereof for detecting abrasion and breakage thereof occurring due to a cutting operation, the signal processing system comprising: means for sampling an output of the abrasion sensor in a predetermined sampling cycle to obtain measurement data; noise eliminating means for eliminating a noise from the obtained measurement data; reduction data separating means for separating reduction data having a time-serially reduced value from the noise-free data; and life expiration determining means for determining an abrasion degree and breakage of the cutting tool on the basis of process data obtained by the separation of the reduction data.
In accordance with a second inventive aspect, the signal processing system according to the first aspect is characterized in that the noise eliminating means comprises median process means for outputting a median of a predetermined number of sampled data.
In accordance with a third inventive aspect, the signal processing system according to the first or second inventive aspect is characterized in that the abrasion sensor comprises a plurality of sensor lines extending parallel to each other along a cutting ridge of the cutting tool, and the life expiration determining means determines the abrasion degree and breakage of the cutting tool by relating the process data to the number of the sensor lines.
In accordance with a fourth inventive aspect, the signal processing system according to the third inventive aspect is characterized in that the life expiration determining means determines an initial resistance value of the plurality of sensor lines on the basis of an initial value of the process data, then determines a resistance threshold which is increased in response to cut-off of each of the sensor lines, and provides an output whenever a value of the process data exceeds the resistance threshold to indicate that the number of the cut-off sensor lines is increased.
In accordance with a fifth inventive aspect, the signal processing system according to any of the first to fourth inventive aspects is characterized in that the cutting tool comprises a throw-away tip provided with the abrasion sensor and a holder for holding the throw-away tip, and the abrasion sensor is capable of detecting the abrasion degree and breakage of the throw-away tip.
In accordance with a sixth inventive aspect, there is provided a signal processing method for a cutting tool having an abrasion sensor provided on a cutting edge thereof for detecting abrasion and breakage thereof occurring due to a cutting operation, the signal processing method comprising the steps of: sampling output data of the abrasion sensor in a predetermined sampling cycle; eliminating a noise from the sampled data; separating reduction data having a time-serially reduced value from the noise-free data; and determining an abrasion degree and breakage of the cutting tool on the basis of process data which is obtained by the separation of the reduction data and is free from time-serial reduction in value.
In accordance with a seventh inventive aspect, the method is characterized in that the abrasion sensor comprises a plurality of sensor lines extending parallel to each other along a cutting ridge of the cutting tool, and the process data has a value which increases time-serially stepwise by increments associated with the number of the sensor lines.
With the arrangements according to the first to sixth inventive aspects, the noise is eliminated from the sampled measurement data. The elimination of the noise can easily be achieved by performing the median process with the use of a median filter or the like as in the second inventive aspect.
The noise-free data is subjected to the data separation process with the use of a separation filter or the like. In the data separation process, the reduction data time-serially reduced in value is separated and eliminated from the noise-free data.
In general, the cutting edge of the cutting tool is abraded as the machining time increases. Since the sensor is accordingly abraded, the resistance increases. However, the reduction data time-serially reduced in value is included in the measurement data. This is supposedly attributable to erroneous data due to action of slugs and the like, because the cutting edge of the cutting tool is brought into contact with a workpiece to be cut thereby to be subjected to the slugs during the cutting operation. Therefore, the slugs of the workpiece hit against the cutting edge of the cutting tool or fuse on the cutting edge, thereby serving to reduce the resistance of the sensor. In the present invention, the data separation process is performed for the elimination of the reduction data time-serially reduced in value, so that only the data having a time-serially increased value is extracted as the process data.
The abrasion degree and breakage of the cutting tool is determined on the basis of the process data thus extracted.
As in the third, fourth and seventh inventive aspects, the determination of the abrasion degree and breakage of the cutting tool can advantageously be achieved by relating the process data to the number of the sensor lines. Where the sensor includes the plurality of sensor lines, the resistance of the sensor lines increases stepwise as the sensor lines are cut off one by one. In the course of the stepwise increase in the value of the process data, the cut-off of each of the sensor lines is detected at a time point at which the increase occurs. Therefore, the number of the sensor lines cut-off at a certain time point of measurement can be determined on the basis of the stepwise change in the resistance, whereby the abrasion degree of the cutting tool can be determined. The expiration of the life of the cutting tool can be detected when the predetermined number of sensor lines are cut off.
The present invention is preferably applied to the throw-away tip as in the fifth inventive aspect. However, the invention is applicable not only to the throw-away tip but also to a cutting tool having no replaceable cutting edge.
A specific embodiment of the present invention will hereinafter be described with reference to the attached drawings.